King Coal is dead, long live the king — in this case, natural gas. President Donald Trump has vowed to reverse coal’s dwindling hold as the fuel of choice. But the trend toward gas is undermining his coal boosterism.

The president recently repealed a regulation, enacted under the Obama administration, that prevented the dumping of coal mining debris into nearby streams. That won cheers in coal regions like West Virginia, where during his election campaign last year, Mr. Trump decried what he called the “war on coal” and blamed “job-killing” environmental rules as the culprit. Indeed, U.S. coal employment, 126,000 in 1940, now is 53,000 and dropping.

While coal usage worldwide for electriciy generation has accelerated over the past quarter-century as less-developed nations expanded their economies, that trend has leveled off lately. And in the U.S., coal-fired generation has dropped 25 percent since 2008. Europe is the midst of a movement to ease away from coal, in keeping with the 134-nation Paris climate accord reached last year. “They just don’t want coal in Europe,” said William Clough, chief executive of CUI Global (CUI), which monitors gas pipelines there…